Spinal deformities, which include rotation, angulation, and/or curvature of the spine, can result from various disorders, including, for example, scoliosis (abnormal curvature in the coronal plane of the spine), kyphosis (backward curvature of the spine), and spondylolisthesis (forward displacement of a lumbar vertebra). Early techniques for correcting such deformities utilized external devices that apply force to the spine in an attempt to reposition the vertebrae. These devices, however, resulted in severe restriction and in some cases immobility of the patient. Furthermore, current external braces have limited ability to correct the deformed spine and typically only prevent progression of the deformity. Thus, to avoid this need, several rod-based techniques were developed to span across multiple vertebrae and force the vertebrae into a desired orientation.
In rod-based techniques, one or more rods are attached to the vertebrae at several fixation sites to progressively correct the spinal deformity. The rods are typically pre-curved intraoperatively to a desired adjusted spinal curvature. Wires as well as bone screws can be used to pull individual vertebra toward the rod. Once the spine has been substantially corrected, the procedure typically requires fusion of the instrumented spinal segments.
While several different rod-based systems have been developed, they tend to be cumbersome, requiring complicated surgical procedures with long operating times to achieve correction. Further, intraoperative adjustment of rod-based systems can be difficult and may result in loss of mechanical properties due to multiple bending operations. The rigidity and permanence of rigid rod-based systems can also hinder or prevent growth of the spine and they generally require fusion of many spine levels, drastically reducing the flexibility of the spine. To help remedy some of these issues, a tether and anchor system can be used to correct curvature of the spine using a number of anchors disposed within the spinal bones connected with tethers extending between them. The elasticity of the tethers prevents some of the problems with the rigidity and permanence of the rod-based systems, although the tethers must be tensioned after implantation to achieve the desired force between the anchor to correct the spinal deformities.
Accordingly, there remains a need for improved methods and devices for correcting spinal deformities and, in particular, there remains a need for methods and devices for tensioning a tether extending between anchors implanted in bone.